Can 'one-way masking' really protect you from COVID on a crowded plane?

·West Coast Correspondent
·7-min read

As soon as a federal judge in Florida struck down the United States' long-standing nationwide mask requirement for airplanes, trains, buses and other public transportation on Monday, COVID-19 experts took to Twitter to reassure uneasy travelers that they still have the power to protect themselves — even when everyone else on board is going gleefully maskless.

“One-way masking works, if you wear the right mask,” tweeted Dr. Scott Gottlieb, former commissioner of the U.S. Food and Drug Administration. “KN95, KF94 or better. For those at risk from Covid, or who want added safety even as those around them unmask, a high quality mask worn properly can afford a measurable degree of added protection.”

“Remember that even if masks aren’t required, people who choose to can wear them (as I will continue to, on planes/trains),” added Dr. Leana Wen, former Baltimore health commissioner. “If you wear one, please opt for an N95 or equivalent (KN95 or KF94) for optimal protection.”

President Biden himself seemed to echo that message Tuesday, when a reporter asked him if Americans should “continue to wear masks on planes” now that most domestic airlines no longer require them.

“That’s up to them,” Biden replied.

A traveler seated in a sparsely populated plane and wearing a surgical mask looks around him.
A traveler at Cairo International Airport in Egypt in a protective face mask in August last year, after an outbreak of COVID-19. (Reuters/Mohamed Abd El Ghany)

The idea of one-way masking isn’t new. Indoor mask mandates have been falling for months, and high-profile voices such as Wen and Gottlieb have responded by insisting that individual masking can still serve as an adequate shield for Americans (especially if they’re fully vaccinated and boosted) who want to minimize their risk of infection during a concert or a trip to the grocery store.

But the sudden toppling of the federal travel mandate — which is unlikely to return — may mark a turning point in the conversation about solo masking. Before, cautious individuals could largely decide not to spend time in places where face coverings were scarce. Many Americans, however, do not have a choice about taking a plane, train, bus or subway to get somewhere. They will now be forced to share air with the great unmasked masses for extended periods — and they’ll want to know whether covering just their own face can really keep the virus at bay.

So what does the latest science say?

Bad news first: Indoor spaces would undoubtedly be safer if every single person were wearing an N95, a KN95 or a KF94 respirator. When properly fitted, these masks prevent about 95% of airborne particles (such as the SARS-CoV-2 virus) from passing through. But because such particles have to breach the mask material twice when everyone is masked up — once after someone breathes them out, then again before someone else breathes them in — universal masking actually doubles the filtration power. As a result, a room, or cabin, of N95s can reduce overall exposure by 99% compared to zero masking.

For anyone who is immunocompromised, ineligible for vaccination or otherwise vulnerable or concerned, that would be the ideal scenario.

The problem, of course, is that it isn’t a realistic scenario, and it wasn’t a realistic scenario even before this week. Travelers on planes, for instance, have long been allowed to remove their masks while eating and drinking. Many, if not most, wear cloth or surgical masks instead of N95s. And even those who do opt for N95-grade respirators tend not to know how to achieve the optimal seal required in, say, laboratories, where professionals undergo a rigorous fit-testing process to minimize leakage. According to a study of “35 untrained, inexperienced subjects” by Lisa Brosseau, a bio-aerosol scientist and industrial hygienist who's a consultant for the University of Minnesota's Center for Infectious Disease Research and Policy, “fewer than 25%” of people were able to get “the fit factor of 100 expected in workplace settings” by themselves.

A masked passenger with one child in a stroller and one following her passes a Covenant Aviation Security officer in a stream of airline passengers.
A Covenant Aviation Security officer monitors airline passengers entering a security checkpoint at San Francisco International Airport on April 19. (Photo by Justin Sullivan/Getty Images)

The question, then, is not how one-way masking compares to perfect and universal N95-grade masking, but rather how one-way masking compares to the patchwork alternative previously in place.

And that’s where the good news comes in. According to Joseph Allen, an associate professor and director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health, a standard cloth mask filters out roughly 50% of virus particles. So “when two people wear this level mask,” Allen explains, “the combined efficacy is 75%.” Meanwhile, a single surgical mask — which uses electrostatically charged, particle-trapping material, like an N95, but doesn’t fit nearly as well — boasts a 70% filtration rate, meaning the combined efficacy of multiple surgical masks is 91%. That’s good enough for hospitals.

Before the travel mandate was lifted this week, mask filtration on planes, trains, buses and subways probably would have fallen somewhere on the lower end of this 75% to 91% range, given how many people were wearing cloth or surgical masks and removing them to eat and drink — or letting them drop under their noses for the entire flight.

But here’s the heartening thing: One well-fitted N95-style respirator is able to filter out 95% of airborne particles all by itself, no matter what anyone else is wearing (or not wearing). Some can perform even better. That means it’s possible for a consistent one-way masker to be even safer in a space where nobody else is masking than a bunch of people wearing a mixture of masks in a space where they’re frequently removing them to snack and sip.

(Airplanes also have air-filtering systems that remove the vast majority of SARS-CoV-2 from the air after takeoff and before landing, which helps protect those who can’t wear N95s for hours on end, like unvaccinated children. These systems should continue to run during boarding and deplaning, experts say, but often don't).

A young woman passenger in a surgical mask seated on a plane.
A traveler wears a protective face mask on a plane at Cairo International Airport in June 2020. (Reuters/Mohamed Abd El Ghany

Fit remains crucial. The Centers for Disease Control and Prevention has posted clear, easy-to-follow instructions on how to get a good fit and test your seal.

“You form the nose clip,” Brosseau explained during a recent NPR interview. “You put both straps over your head. You don't have big gaps under your chin or around your nose. You can be assured that you're probably going to get pretty good protection.”

It’s also important to avoid counterfeits. For that, try shopping through Project N95, a nonprofit that helps consumers find legitimate masks and respirators, or consulting this extensive database of mask test results drawn up by a mechanical engineer, Aaron Collins. And to lessen inequities and improve access in the post-mandate era, Wen suggests that the federal government should make "N95/KN95s free for all who want them."

Even a real, properly fitted, one-way N95 can’t completely protect its user forever. According to a modeling study published in the Proceedings of the National Academy of Sciences in December, someone who wears a well-fitting European N95 counterpart respirator has a 20% risk of catching COVID after talking to a contagious person at a distance of 5 feet for an hour.

Yet when both parties are wearing surgical masks, that risk increases to nearly 30% — again suggesting that while solo N95 masking is not as foolproof as universal N95 masking (risk: 0.4%) it can be even more effective for concerned individuals than the existing standard in hospitals, or the messy status quo that prevailed on planes, trains, buses and subways before this week.

If you are vaccinated, boosted and sporting a well-fitted N95 (or similar) indoors, “your risk is extremely low,” Allen told The Atlantic back in January. “I mean, there’s not much else in life that would have as low a risk as that.”

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